1. FIELD OF THE INVENTION
The field of the invention is catalytic cracking of heavy hydrocarbon feeds using a moving or fluidized bed of cracking catalyst.
2. DESCRIPTION OF RELATED ART
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular and efficient fluidized bed process.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone much development since the 40s. The trend has been from moving bed cracking to fluid bed cracking. The trend in fluid catalytic cracking (FCC) has been to all riser cracking and use of zeolite catalysts.
Zeolite-containing catalysts having high activity and selectivity are now used in most FCC units. These catalysts work best when coke on the catalyst after regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %.
To regenerate FCC catalysts to these low residual carbon levels, and to burn CO completely to CO2 within the regenerator (to conserve heat and minimize air pollution) many FCC operators add a CO combustion promoter metal to the catalyst or to the regenerator.
U.S. Pat. Nos. 4,072,600 and 4,093,535, which are incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Modern, zeolite based catalysts are so active that the feed can be cracked to lighter, more valuable products in less time. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, less contact time is needed. Conversion of feed can now be more efficiently achieved in a dilute phase, riser reactor.
As the process and catalyst improved, refiners used the process to upgrade heavier feedstocks.
These heavier, dirtier feeds placed a growing demand on the reactor and on the regenerator. Processing resids exacerbated existing problem areas in the riser reactor, namely feed vaporization, catalyst oil contact, accommodation of large molar volumes in the riser, and coking in the transfer line from the reactor to the main fractionator. Each of these problem areas will be briefly discussed.
Feed vaporization is a severe problem with heavy feeds such as resids. The heavy feeds are viscous and difficult to preheat in conventional preheaters. Most of the heating and vaporization of these feeds occurs in the base of the riser reactor, where feed contacts hot, regenerated catalyst. Because of the high boiling point, and high viscosity, of heavy feed, feed vaporization takes longer in the riser, and much of the riser length is wasted in simply vaporizing feed. Multiple feed nozzles, fog forming nozzles, etc., all help some, but most refiners simply add more atomizing steam. Use of large amounts of atomizing steam helps produce smaller sized feed droplets in the riser, and these smaller sized drops are more readily vaporized. With some resids, operation with 3-5 wt % steam, or even more, approaching in some instances 5-10 wt % of the resid feed, is needed to get adequate atomization of resid. All this steam helps vaporize the feed, but wastes energy because the steam is heated and later condensed. It also adds a lot of moles of material to the riser. The volume of steam approaches that of the volume of the vaporized resid in the base of the riser. This means that up to half of the riser volume is devoted to steaming (and deactivating) the catalyst, rather than cracking the feed.
In U.S. Pat. No. 4,427,537, which is incorporated herein by reference, higher temperature cracking of resids was advocated. The resid was added to the base of a riser reactor as a fog of oil droplets in diluent. High temperatures in the base of the riser, at least equal to or above the pseudo critical temperature of the oil feed, caused a thermal shock which promoted thermal cracking of large molecules into smaller molecules which could be recracked catalytically in the gas phase.
In U.S. Pat. No. 4,816,137, which is incorporated herein by reference, high temperature cracking of resids was achieved using a two stage regenerator which produced extremely hot regenerated catalyst. The first regeneration stage operated at 1150.degree.-1500.degree. F., in partial CO combustion mode, to produce a flue gas comprising large amounts of CO. This first stage of regeneration removed most of the hydrogen from the coke on catalyst. Regeneration was then completed in a second stage of regeneration at 1400.degree.-1800.degree. F. The second stage of regeneration was isolated from the first stage, and the second stage operated relatively dry, so the catalyst was not steamed as much in the second stage.
This approach required construction of a special, multi-stage regenerator, with external, refractory lined cyclones on the second stage, but it was a way to get the very hot catalyst required to vaporize feeds containing 10 or 20 wt % or more of resid. The hydrocarbon feed was preheated to a temperature below 800.degree. F., then mixed with enough very hot catalyst to achieve a mix temperature above the feed pseudo-critical temperature.
In U.S. Pat. No. 4,818,372, which is incorporated herein by reference, high temperature thermal cracking of resid in the base of a riser was followed by quenching and catalytic cracking in the riser. Enough extremely hot catalyst was added to a resid feed, which was preheated to 150.degree. C., to achieve a mix temperature in the base of a riser of 595.degree. C. The patentee recognized that higher feed preheat temperatures would improve feed vaporization, but the highest feed preheat temperature reported was 150.degree. C.
Although the general approach of U.S. Pat. No. 4,816,137 and U.S. Pat. No. 4,818,372 would help vaporize resids, it might make downstream processing more difficult, particularly if a feed with a high coking tendency was being cracked.
Coking in the transfer lines and fractionation columns associated with the cracking reactors is a problem in many catalytic cracking units. FCC operators have long known that "dead spaces" in a line or a process vessel could lead to coke formation. Coke formation is a frequently encountered problem in the "dome" or large weldcap which forms the top of the vessel housing the riser reactor cyclones. If oil at high temperature is allowed to remain stagnant for a long time, it will slowly form coke. For this reason refiners have routinely added a small amount of "dome steam", typically 500 #/hr, to prevent formation of coke in the dome of an FCC unit. Coking in the transfer line is somewhat related, in that coke will form in stagnant or dead areas of the transfer line.
Coke will also form if there are cool spots in the transfer line. The cool spots allow some of the heaviest material in the reactor effluent vapor to condense. These heavy materials, some of which may be entrained asphaltenic materials, will form coke if allowed to remain for a long time in the transfer line. Thus refiners have tried to insulate the transfer line to the main column, not only to prevent heat loss to the atmosphere, but also to prevent coking in this line.
Coking in fractionators has sometimes been a problem, particularly in fractionators associated with older cracking units such as moving bed catalytic cracking (TCC) units. Some TCC units have a quench zone at the inlet of the main column associated with the TCC reactor. A heavy liquid is recycled from the column to contact hot vapor feed from the cracking reactor, and cool it enough so that a two phase mixture would enter main column. This prevented coke formation in stagnant spaces on the main column.
The problem of coke formation in transfer lines or fractionation columns gets more severe with either an increase in reactor/transfer line temperatures, or with a decrease in feed quality so that it contains more heavier materials.
It would be beneficial if a higher mix temperature could be achieved in the base of a riser reactor with lower catalyst temperatures. High temperatures are hard on the catalyst, even in a relatively dry atmosphere.
It would be beneficial if the vaporizability of the resid containing feeds could be substantially improved, without use of excessive amounts of atomizing steam or inclusion of large amounts of hydrocarbon diluents in the resid feed. Inclusion of 5 or 10 wt % steam with the resid, or added as atomizing steam, will improve vaporization, but the steam will take up half of the volume of the riser and downstream processing equipment.
I realized that the most efficient way to improve the properties of a resid feed for cat cracking was to heat it. Breaking through the traditional feed preheat limit of 300.degree.-600.degree. F. used in most cat crackers, and the 800.degree. F. limit mentioned in U.S. Pat. No. 4,816,137, translates into a more pumpable, more atomizable feed.
I knew that if higher feed preheat could be achieved, the lighter feed components would function to a great extent as solvents and/or atomizing diluents.
I wanted to avoid, however, completely vaporizing a heavy feed upstream of the cracking unit. Running a coker upstream of the catalytic cracking unit, and charging only a vapor phase to the cat cracker will reduce and perhaps even eliminate the CCR from the cat cracker feed, but there is a significant yield penalty associated with a coker. The art has long recognized that the products of thermal cracking, coking or visbreaking are not particularly good feed stocks for FCC units due to resulting high concentrations of polynuclear aromatics. This is discussed in U.S. Pat. No. 4,816,137 in the review of the prior art.
Thus the art has gone in several directions in treating distress stocks such as resids. Extremely high temperatures achieved via hot catalyst or lower temperatures with a vapor feed.
The approach exemplified by U.S. Pat. No. 4,816,137 leaves the feed preheat alone, and resorts to extremely high temperature catalyst. That approach is hard on the catalyst, and uses high grade heat (1400.degree. F. catalyst) to heat a low temperature stream (300.degree. to 450.degree. F. resid feed).
The other approach, coking the feed first, or use a two stage fluidized bed unit, with the first stage acting as a coker and cascading the vapor product into a second stage acting as a catalytic cracking unit, costs a lot in equipment and operating expense, and loses too much yield.
I knew that neither approach (cool liquid feed, or vapor feed from coking) was a complete solution. The approach suggested in PCT/U.S./ No. 87 0227, published as WO No. 88/01638 on Mar. 10, 1988, seemed a better solution. This approach called for fairly severe, liquid phase thermal treatment of the feed, as by visbreaking, and cascading the freshly visbroken feed into the catalytic cracking unit. In the laboratory tests reported in the patent publication, use of extremely high preheat temperature improved the octane number of the gasoline produced by cracking an Arab Light atmospheric resid.
The PCT publication suggested that cascading the liquid effluent from a visbreaking unit into an FCC would improve the cracking process. Although visbreaking is not especially complicated or expensive, it requires use of fired heaters to get the high temperatures needed for the visbreaking reactions to occur.
From the above review of cat cracking developments, it can be seen that the trend is to heavy feeds. To catalytically crack, rather than coke, heavy viscous feeds, refiners have resorted to high temperatures and high vapor velocity in the riser, and tall risers. This helped improve the cracking process and allowed FCC units to process significantly heavier feeds. Unfortunately, although the catalyst could keep pace with the heavier feed, the process could not. It became harder to heat feeds as they became heavier. To compensate, the regenerator had to run hotter. The higher catalyst temperatures, and higher operating temperatures need to crack the poor quality, high CCR feed led to thermal cracking in the cracking unit, and coking in transfer lines downstream of the unit.
Bottlenecks were developing both upstream and downstream of the cracking units. The heavier feeds were hard to vaporize in the cracking reactor. The hot products of cracking were coking the downstream processing equipment.
I examined the work that others had done, and realized that it was time for a new approach. I wanted to crack resid, but did not want to have to run the regenerator at extraordinarily high temperatures. I knew that the problems of metals contamination (Ni+V) were more severe at high regenerator temperatures, and that there had to be a better way to crack resid than shocking it with extremely hot catalyst. I did not want to tie up 20-50% of my reactor volume with atomizing steam, steam that was primarily added to promote dispersion and vaporization of "non-distillable" resid feed.
I wanted a way to take more heat out of the cracking reaction zone. If the reactor could be run hotter, and the cracked vapor leaving the reactor could be removed at a higher temperature, then more heat could be removed from the cracking unit, as compared to prior art units which operated with an upper limit of about 1000.degree. F. for the cracked vapor from the reactor.
I realized that there was a way to overcome the limitations of existing technology. By using the hot reactor vapor to preheat the heavy feed I could efficiently preheat heavy feeds, remove more heat from the reactor and bring the FCC back into better heat balance, improve feed vaporization in the riser, and eliminate thermal reactions and coking in the transfer line to the product fractionators.